Adaptive Engine Technologies for Aviation CO2 Emissions Reduction

NASA/TM—2006-214392 1 CO2. It should be noted that NOX generation is a function of the combustor design, and may or may not decrease with decreased fuel burn. This paper contains a brief description of some representative adaptive turbine engine technologies, followed by a description of the methodology used to assess their benefit for emissions reduction. A table is presented showing potential emissions reduction for each as a function of vehicle class. II. Adaptive Engine Technologies There are numerous technologies that are under development to adaptively modify turbine engine performance. These adaptive technologies can lead to improved engine component efficiency and/or reduced weight, both resulting in overall fuel burn reduction. As a rule of thumb, for a large subsonic aircraft a 1000 pound reduction in weight yields a 0.5-0.7% reduction in jet fuel consumed. For carbon based fuels, there is a 1:1 relationship between the amount of fuel burned and the amount of CO2 generated. Alternative fuels were not considered. The primary classes of adaptive technologies are flow control, structural control, combustion control, and also enabling technologies that are applicable to each. Representative technologies from each of these classes are briefly described below. A. Flow Control Flow control technologies directly manipulate air flow through or around a specific engine component. The manipulation is enacted by actively injecting or extracting air, by inserting small mechanical protuberances into the flow, or by using plasma actuators. Injected air can be supplied by bleed from a rear compressor stage, or by forming “synthetic” jets from a local cavity with an oscillating membrane that cyclically entrains and discharges air. Air injection is then used to energize low momentum regions within the main flow. The protuberances can be actively inserted and retracted based on flow conditions, or they can be designed to passively react to the flow; in both cases the intent is to influence boundary layer separation. Plasma actuators employ electrical actuation rather than pneumatic. Subsonic inlet flow control can be used, for example, to maintain performance, engine stability, and engine durability under a variety of flow conditions. This leads to shorter and more conformal inlet designs with reduced weight. Since for a large subsonic aircraft engine, inlets can contribute about 10% of the engine system weight, this reduction can be substantial. For supersonic inlets, bleed drag is often found to be the most significant component of inlet drag at cruise. Therefore, eliminating the bleed drag and the weight and complexity of the bleed system is a major thrust in modern flow control for supersonic inlet design. Similarly, fan flow control enables the fan to operate with poorer quality flow from the inlet. This can allow a tighter integration of the inlet and fan, providing good performance over a wide range of operating conditions with a shorter, lighter inlet. Flow control can be used to improve compressor performance by sensing pressure disturbances preceding flow separation, then energizing the air ahead of the separation line. Flow can be controlled through the airfoil to improve flow quality, and in the end-wall region to enable safe compressor operation at reduced stall margins. Both offer the potential to increase aerodynamic loading per blade without reducing aerodynamic efficiency, and thus offer the promise of reducing the number of airfoils (and therefore compressor weight) needed to achieve a given pressure ratio. 6,7 Reduced stall margins can also enable compressor operation closer to the peak efficiency operating point. For a large subsonic aircraft engine, compressor stages can be 15% of the engine’s weight, and a 1% improvement in high-pressure compressor efficiency can lead to 2% reductions in fuel burn. Flow control can be used to cool structures as well, such as closed-loop cooling control for turbine blades. By sensing hot-spots as they occur and only cooling as necessary, the total mass of bleed air can be reduced. Bleeding air from the compressor directly reduces the percentage of inlet air available for combustion, so bleed air reduction translates directly into propulsion efficiency improvement. B. Structural Control Actively controlling the clearances between rotating blades and shrouds directly improves fan, compressor, and turbine efficiency by reducing leakage through the clearances at each stage. Current engines are designed with sufficient clearance to minimize rubbing during flight. Typically these clearances are sized to prevent rubbing during take-off, and are thus larger than necessary during cruise. Excess clearance allows leakage through the gap, diverting air away from its intended path through the core or bypass ducts. Current open-loop clearance control systems use compressor and/or fan bleed air to cool the case during cruise and therefore close the gap. Closed-loop clearance control promises finer control of the gap while preventing rub-induced component degradation. For a NASA/TM—2006-214392 2 large subsonic aircraft engine, each 10 mils of excess clearance increases specific fuel consumption by roughly 1%. This will require an increase in exhaust gas temperature margins by about 10 °C, in order to maintain the same engine thrust level. The ability to maintain tight clearances can provide both a substantial fuel-burn reduction and increased engine life. These closed-loop active clearance control systems require robust, accurate and precise sensors and actuators. High-temperature, high-loading magnetic bearings and self-tuning vibration absorbers for engine blades can be used to adaptively control structural vibrations. Conical magnetic bearings can also be used for active compressor stall control. Prime reliant magnetic bearings can eliminate the need for existing oil systems, reducing the weight of engine peripherals. However, weight penalty can be large if auxiliary bearings are needed to handle blade-out load and as safety backup, in addition to the weight of the electrical power and control systems required for operation. Variable-area fan nozzles have been considered to enable low fan-pressure-ratio, high bypass-ratio thermodynamic cycles that operate well during both low speed operation (take-off and landing) and high speed cruise. These cycles improve propulsion efficiency, and therefore reduce fuel burn and emissions, although their benefits diminish with increasing fan pressure-ratio. Shape memory alloys have been investigated to provide up to 20% nozzle area variability, and are substantially lighter than conventional hydraulic actuators. C. Combustion Control Combustion control technologies are being developed to both enable lean-burning combustors and to directly control the local combustion process thus providing more uniformly efficient burning. A new generation of leanburning combustors is being developed to reduce emissions, but they are more susceptible to combustion instability and flame-out. Active combustion control provides closed-loop, dynamic control of fuel injection, fuel air mixing, and fuel source staging to disrupt the coupling between the combustion process and combustor acoustics leading to instabilities. Pressure sensors are used to monitor the combustor acoustics, and control laws are used to dynamically modulate high-response-rate actuators in the fuel line. To achieve uniform burning, sensor arrays determine the planar cross-sectional temperature distribution to drive actuators in individual fuel injectors. The larger the number of fuel injectors, the finer the control of the spatial distribution. “Pattern factor” control is also being investigated to produce spatially uniform combustion, eliminating hot and cold spots that generate NOX and CO2 emissions, respectively. Sensors determine either the local temperature distribution across a cross-section of the combustor, or sense emissions directly for use in closed-loop fuel injector control. D. Enabling Technologies Adaptive control can be either active or passive. Passive techniques include self-triggered mechanisms such as thermally-triggered shape memory alloys or microstructures triggering flow disturbances after a specific velocity has been reached. Active techniques require at a minimum a sensor, control logic, and an actuator. To achieve these, some subset of sensors, electronics, materials, actuators, wireless communications, power generation, and control logic are required. These technologies do not reduce emissions on their own, but they are critical for the practical embodiment of the aforementioned flow, structural, and combustion control technologies that directly reduce emissions. Specific sensors of use for adaptive engine components include: temperature and pressure sensors (both static and dynamic), surface and gas; mass flow, surface strain, and blade tip clearance sensors. Applications exist for each of these sensors throughout the engine, including the hot sections of the turbine and nozzle. In addition, specialized sensors for the combustor include fuel flow, chemical species, and temperature sensors that can withstand high temperatures (typically 1000 °C) and can operate in the presence of by-products from burning jet fuel. Not only the sensors need to operate at elevated temperatures; each sensor system typically includes processing electronics, and weight is reduced (hence fuel-burn reduced) by using wireless communications and locally-scavenged power. Actuators are needed for flow control in the inlet, fan, compressor, and turbine; clearance control in the compressor and turbine; and for fuel modulation. Desirable actuator characteristics include fast response times, low weight and bulk, and reliable operation in the engine environment. Active materials such as piezoelectric and shape memory alloys can be used as both actuators and sensors, including in the hot sections.